WASHINGTON -- Facing strong pressure from a coalition of auto makers, union workers and business groups, the Senate appears to be turning away from a proposal to raise the average fuel efficiency of U.S. cars and trucks to as high as 36 miles per gallon from the current 24 mpg.

After a day of maneuvering over the broader energy bill, a substitute measure sponsored by Sens. Carl Levin (D., Mich.) and Christopher Bond (R., Mo.) appeared to pull enough Democrats from industrial states into a Republican-led coalition to defeat...